This invention generally relates to a system and method that enables an eddy current probe to determine the proximity of non-conductive structures, and the dimensions of both non-conductive and non-magnetic structures by the use of a portable target medium.
Eddy current probes for inspecting the condition of structures formed from conductive materials are well known in the prior art. Such probes generally comprise a sensor coil, a multi-frequency generator for conducting high frequency alternating current through the coil in order to generate a fluctuating electromagnetic field, and a circuit for measuring the amount of impedance experienced by an alternating current as it flows through the coil windings. When the sensor coil of such an eddy current probe is placed in the vicinity of a structure formed from a conductive material, the electromagnetic field emanated from the coil couples with the conductive material and induces a counter-flowing alternating magnetic field in the material. This counter-flowing magnetic field in turn induces eddy currents in the material. The counter-flowing electromagnetic field and eddy currents imposes an impedance on the electromagnetic field emanated by the sensor coil which may be precisely measured by the impedance measuring circuit of the probe. In some applications, the measured impedance applied to the sensor coil of the probe is used merely to detect the presence or absence of a particular conductive structure, such as the presence of metallic objects buried in the sands of a beach. In other applications, the measured impedance is used to detect the presence or absence of structural faults such as cracks, pits, or areas of wall thinning in a metal pipe or other conductive structure. In all applications, the impedance experienced by the fluctuating electromagnetic field emanated by the sensor coil measurably changes as the coil is scanned around the vicinity of a conductive structure, both as a function of the distance from the sensor coil to the structure, and as a function of physical variations in the structure, such as changes in wall thickness, the presence or absence of current-impeding cracks or other faults in the structure, or changes in the conductivity of the material used to form the structure. Because of the need in such areas as the nuclear engineering arts to be able to accurately and remotely inspect the structural integrity of reactor and steam generator components in hostile environments, a large body of sophisticated knowledge and expertise has been developed that is aimed at extracting detailed information about a metallic structure being scanned by an eddy current probe.
Unfortunately, eddy current probes cannot be used to detect or inspect structures formed from nonconductive materials, since the fluctuating electromagnetic field emanated by the coil sensor cannot couple with non-conductive materials. While there are alternate modes of inspection such as ultrasonic probes which do not require the structure being examined to be formed of electrically conductive materials, these alternate inspection modes are sometimes difficult if not impossible to implement. For example, the use of ultrasonic inspection probes requires the presence of a liquid couplant, such as water, between the probe head and the structure being inspected. In certain applications, such as the inspection of remote components of electrodynamic machinery, it may be highly undesirable, if not impossible, to provide such a liquid couplant around the structure. Moreover, even in instances where the nature of the structure or its accessibility or its environment does not pose a major obstruction to the application of a liquid couplant around the structure, there are some non-conductive structural materials that are inherently uninspectable by ultrasonic probes, such as porous ceramics. Any such liquid couplant would penetrate and be retained by the porous nature of such ceramics. Thus, there is no known satisfactory technique for inspecting the walls or measuring the wall thicknesses of the small diameter, thin walled porous ceramic tubing used for fuel cell and gas filtering applications.
Additionally, even in the case of structures which are conductive but not formed of non-magnetic materials, there are instances where neither eddy current nor ultrasonic inspections are capable of accurately determining the dimensions of such structures. For example, in assessing the wall thickness variations in the Zircaloy.RTM. guide tubes used in nuclear fuel assemblies, it is possible for an eddy current probe to yield inaccurate results due to differences in conductivity along the length of the tube caused by differences in the orientation of the zirconium crystals. Additionally, an ultrasonic probe is not satisfactorily accurate across the ten foot length of such tubes because the axial taper present within the small diameter of such tubes (which have an outer diameter of only about 0.50 inches) prevents sufficient control of the interrogating sound beam.
Clearly, both a system and method are needed for both detecting the presence and for measuring the dimensions of remotely-located structures formed from non-conductive materials which is as accurate and reliable as the state-of-the art eddy current probe inspections made of structures formed from conductive materials. Ideally, a system and method would allow such inspections to be performed easily, cheaply, and remotely and would make maximum use of commercially available inspection equipment. Finally, it would be desirable if such a system and method were capable of accurately measuring the dimensions of any structure made from any non-conductive or non-magnetic material, and in particular structures formed from non-conductive, non-magnetic materials where an ultrasonic or eddy current probe is either incapable of achieving accurate results, or highly undesirable or impossible to apply.